What an unpleasant man!
You will fit into the 24 month rule as you will be based at the temporary location for, say 3 months, which is a very small percentage of 24. You will not be based there during your maternity leave - this is a temporary adjustment being made to accommodate your pregnancy due to Health and Safety concerns which no longer apply once you are on maternity leave.
Don't feel you need to be apologetic, he should be extremely careful speaking to you like that, he is obviously a bit peed off that you are pregnant and is not concealing it very well, most unprofessional as you say.
I'm not a tax expert by any means. My initial instinct with this was you shouldn't be out of pocket, my (limited) tax knowledge told me that travel expenses to a normal place of work are taxable, which I then posted along with everyone else.
But my instinct kept nagging me and I vaguely remembered something about temporary workplaces so I had a look and found the link which is obviously what you found as well. I don't think it's a 'grey area' as such, but something that isn't immediately obvious until you look into it. You have helpfully done this for your boss but I think he's just in a bit of a mood generally and doesn't want to know. He also is probably one of those people who doesn't like being proved wrong, so when someone corrects him he would rather say 'it's a grey area' than say 'oh I'm not sure actually, you may well be right, I'll check, thank you for bringing it to my attention'.
I'd consider reiterating all this in an email, cut and paste the relevant section from the IR website and attach the link as well, so he can read it in his own time. Write the email fairly formally and non-apologetically.
Who does your payroll, is it inhouse, someone you can speak to? If they are aware of this rule or you can point them to it, your boss might be more inclined to listen to them. Or if payroll is outsourced maybe you know someone in Finance who could verify to him for you?